Pressure control system for suspension

ABSTRACT

A suspension pressure control system on a vehicle applies a pressure which is proportional to a current level supplied to a solenoid of a pressure control valve to a shock absorber associated with respective one of suspensions from the valve. An electronic controller controls the current level to maintain an attitude of a vehicle substantially constant in spite of any change of longitudinal and lateral acceleration, which are detected by acceleration sensors. If a driver on the vehicle closes test indication switches TSW1 and TSW2, indicates &#34;high&#34; by a height indication switch HSW and rotates a steering wheel on the vehicle by B toward right, the vehicle laterally inclines such that a height of the left side of the vehicle rises up and a right side falls down responding to the rotation of the steering wheel. The inclination increases as the rotation of the steering wheel increases. The inclination decreases when the steering wheel rotates inverse direction toward the neutral position. When the steering wheel rotates beyond the neutral position toward left, the vehicle laterally inclines such that the height of the left side of the vehicle calls down and the right side rises up responding to the rotation of the steering wheel.

FIELD OF THE INVENTION

The invention relates to a pressure control of vehicular suspension, inparticular, to a system which controls a suspension pressure in a mannerto suppress a change in the attitude of a car body as caused by asteering operation or acceleration/deceleration thereof.

Japanese Laid-Open Patent Application No. 106,133/1988 discloses apressure control system in which a turning pattern of a vehicle isdetermined on the basis of a steering angle and a steering angularvelocity, and is utilized to modify a constant of proportionality orgain which is to be applied to a corrected suspension pressure, and arequired suspension pressure is calculated in accordance with a gain anda lateral acceleration of the vehicle so as to supply the requiredpressure to the suspension.

Japanese Laid-Open Utility Model Patent Application No. 202,404/1987discloses an attitude control device which detects a height of a carbody with a height detector and controls a pressure of a suspensionresponding to the height detected. There are many other well knownheight control systems.

The height or attitude of the car body may become abnormal if a sensorfor detecting a running condition of the car, a pressure determiningdevice, an electronic circuit or computer for controlling the pressureof the suspension through the pressure determining device responding tothe running condition detected fails.

A height control system in the prior arts includes a height indicationswitch for indicating "high" or "normal". A driver may test a failure ofthe system by switching the indication through the switch and monitoringwhether the height of the car increases or decreases as a result of theswitching. The height of the car body at each of the wheels (four:4)increases or decreases at the same time when the system is normal.However, a failure of a height control of a suspension at a wheel maynot be detected because the height at the wheel may increase ordecreases when the remaining height controls at each of the remainingthree wheels are normal and thus the car body is supported by theremaining three suspensions.

The control system discloses in the Japanese Laid-Open PatentApplication No. 106,133/1988 controls the pressure of the right sidesuspension and the left side suspension separately, responding to asteering speed for suppressing a rolling of the car body which may ariseby steering. A failure of a steering detector and a failure of thepressure control responding to the steering speed may not be detected bythe test described above.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

The present patent application is one of a group of copending patentapplications which concern the same overall pressure control system ormodifications thereof but which individually claim different inventiveconcepts embodied. These related patent applications are specificallyincorporated by reference herein, and are more particularly described asfollows:

(1) Application Ser. No. 458,240 entitled "Pressure Control System forSuspension", filed on Dec. 27, 1989, the inventors being Messrs. OsamuKomazawa et al.;

(2) Application Ser. No. 457,957 entitled "Pressure Control System forSuspension", filed on Dec. 27, 1989, the inventors being Messrs. KouichiKokubo et al.;

(3) Application Ser. No. 475,268 entitled "Pressure Control System forSuspension", filed on Feb. 5, 1990, the inventors being Messrs. TsukasaWatanabe et al.;

(4) Application Ser. No. 503,627 entitled "Pressure Control ValveDevice", filed on Apr. 3, 1990, the inventors being Messrs. MasahiroFukuta et al.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a pressure control systemwhich can be easily tested by a driver of a vehicle for detectingcorrectly a failure of a suspension control.

A pressure control system of the invention comprises pressuredetermining devices, each of which determines, in response to a pressureinstruction, a pressure of one of the suspensions on a vehicle, sensorsfor detecting running conditions of the vehicle and an electroniccontroller for calculating pressures in response to the runningconditions detected and instructing pressures to each of the pressuredetermining devices. The system of the invention further comprises testindication switches for indicating test controls of the suspensions tothe electronic controller and a test pressure indicator which indicatestest pressures for the suspensions to the electronic controller.

The electronic controller, responding to an indication of a first testcontrol from the test indication switches, generates a first pressureinstruction for causing a pressure difference, which corresponds to atest pressure indicated by the test pressure indicator, between thesuspensions at the front side and the rear side. The electroniccontroller, responding to an indication of a second control from thetest indication switches, generates a second pressure instruction forcausing a pressure difference, which corresponds to a test pressureindicated by the test pressure indicator, between the suspensions at theright side and the left side.

According to the invention, a forward or backward inclination (pitching)of the vehicle arises when a driver on the vehicle operates the testswitches so as to generates the first control instruction. Theinclination increases when the driver operates the test pressureindicator so as to generates a higher pressure. If a pressure control ofa suspension should fail, there will arise a rightward or leftwardinclination (rolling) in addition to the pitching. A rightware orleftware inclination (rolling) of the vehicle arises when a driver ofthe vehicle operates the test switches so as to generates the secondcontrol instruction. The inclination increases when the driver operatesthe test pressure indicator so as to generates a higher pressure. If apressure control of a suspension should fail, there will arise a forwardor backward inclination (pitching) in addition to the rolling. Thus thedriver may test the operation of the system of the invention and maydetermine a failure of a pressure control of a suspension by monitoringwhether there arise an normal change of the attitude of the vehicle ornot.

Other objects and features of the invention will become apparent fromthe following description of an embodiment thereof with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are block diagrams of a suspension pressure feedingsystem according to a first embodiment of the invention, each Figurerepresenting one of halves of a single system;

FIG. 2 is a longitudinal section, to an enlarged scale, of a suspension100fr shown in FIG. 1a;

FIG. 3 is a longitudinal section, to an enlarged scale, of a pressurecontrol valve 80fr shown in FIG. 1a;

FIG. 4a and 4b are block diagrams of an electrical control system whichcontrols a suspension pressure in response to values detected by avehicle elevation sensor, pressure sensor or the like, of the suspensionpressure feeding system shown in FIGS. 1a and 1b, both Figures beinghalves of a single electrical control system;

FIGS. 5a, 5b and 5c are flow charts illustrating a control operation bya microprocessor 17 shown in FIG. 4a;

FIGS. 6a, 6b, 6c, 6d, 6e, 6f, 6g, 6h and 6i are flow charts showingsubroutines shown in FIG. 5c in detail; and

FIGS. 7a and 7b graphically show data content which is written into aninternal ROM of CPU 17 shown in FIG. 4a.

DESCRIPTION OF PREFERRED EMBODIMENT

FIGS. 1a and 1b show a mechanical arrangement of an apparatus whichsupports a car chassis or car body. An oil pressure pump 1, which is ofradial type, is disposed within an engine room, and is driven forrotation by an onboard engine, not shown, thus drawing an oil from areservoir 2 and discharging a given flow rate of oil to a high pressureport 3 at a rotational speed equal to or greater than a given value.

High pressure port 3 of the radial pump is connected to an accumulator 4which serves suppressing pulsations, to a main check valve 50 and to arelief valve 60m. A high pressure oil from the port 3 is fed to a highpressure piping 8 through the valve 50. The check valve 50 blocks areverse flow of the oil from the piping 8 to the port 3 whenever theport 3 assumes a lower pressure than the piping 8. The relief valve 60mdrains the port 3 into a reservoir return piping 11, which representsone of the port 3 assumes a greater than a given value, thus maintainingthe pressure of the port 3 substantially at a given pressure.

The high pressure feed piping 8 communicates with a front wheel highpressure feed piping 6 which feeds a high pressure to front wheelsuspensions 100fL, 100fr, and a rear wheel high pressure feed piping 9which feeds high pressure to rear wheel suspensions 100rL, 100rr. Thepiping 6 communicates with an accumulator 7 for the front wheels whilethe piping 9 communicates with an accumulator 10 for the rear wheels.

The piping 6 is also connected to a pressure control valve 80fr throughan oil filter, and the valve 80fr is effective at or regulating thepressure from the piping 6, which will be hereinafter referred to as afront wheel line pressure, to a required pressure before supplying it toa cut valve 70fr and a relief valve 60fr. The required pressure issubstantially proportional to a current level which is used to energizean electrical coil of the valve 80fr and represents a suspension sustainpressure.

When the pressure of the piping 6 or the front wheel line pressure isbelow a given value, the cut valve 70fr interrupts a communicationbetween the output port 84 (to the suspension) of the pressure controlvalve 80fr and a hollow piston rod 102fr associated with a shockabsorber 101fr of the suspension 100fr, thus preventing the pressure ofthe piston rod 102fr from being released to the pressure control valve80fr. When the front rear line pressure remains at or above the givenvalue, the cut valve 70fr allows the output pressure from the pressurecontrol valve 80fr to be directly fed to the piston rod 102fr.

The relief valve 60fr restricts the internal pressure of the shockabsorber 101fr at or below an upper limit. Specifically, when thesuspension sustain pressure from the output port 84 of the pressurecontrol valve 80fr exceeds a predetermined high pressure, the outputport 84 is drained to the reservoir return path or piping 11, thusmaintaining the output port of the valve 80fr substantially at or belowthe predetermined high pressure. The relief valve 60fr is also effectiveto provide a buffering action upon the transmission to the pressurecontrol valve 80fr of any shocking rise in the internal pressure of theshock absorber 101fr, as when a front, right wheel of the vehiclestrikes against a bump on the road. In response to such rise, theinternal pressure of the shock absorber 101fr is drained to thereservoir return path 11 through the piston rod 100fr and the cut valve.

The suspension 100fr essentially comprises the shock absorber 101fr anda coiled suspension spring 119fr, and operates to maintain a car body atan elevation relative to the front, right wheel, which corresponds tothe pressure supplied to the shock absorber 101fr from the output port84 of the pressure control valve 80fr through the piston rod 102fr, or apressure which is regulated by the pressure control valve 80fr or thesuspension sustain pressure.

The sustain pressure fed to the shock absorber 101fr is detected by apressure sensor 13fr, which produces an analog signal representing adetected sustain pressure. A vehicle elevation sensor 15fr is mounted onthe car body adjacent to the suspension 100fr, and includes a rotorconnected to a link which is coupled to the front, right wheel. In thismanner, the sensor 15fr produces digital data representing the elevationof a car body with respect to the front, right wheel.

In a similar manner, a suspension 100fL associated with a front, leftwheel is equipped with a pressure control valve 80fL, a cut valve 70fL,a relief valve 60fL, a vehicle elevation sensor 15fL, and a pressuresensor 13fL. The pressure control valve 80fL is connected to the frontwheel high pressure feed piping 6, thus feeding a required pressure tothe piston rod 102fL of the shock absorber 101fL of the suspension100fL.

Furthermore, a suspension 100rr associated with a rear, right wheel isequipped with a pressure control valve 80rr, a cut valve 70rr, a reliefvalve 60rr, a vehicle elevation sensor 15rr and a pressure sensor 13rr.The valve 80rr is connected to the rear wheel high pressure feed piping9, feeding a required pressure to the piston rod 102rr of a shockabsorber 101rr of the suspension 100rr.

Finally, a suspension 100rL associated with a rear, left wheel isequipped with a pressure control valve 80rL, a cut valve 70rL, a reliefvalve 60rL, a vehicle elevation sensor 15rL and a pressure sensor 13rL.The pressure control valve 80rL is connected to the rear wheel highpressure feed piping 9, feeding a required pressure to the piston rod102rL of a shock absorber 101rL of the suspension 100rL.

In this embodiment, an engine is mounted on the front wheel side, andaccordingly the pump 1 is also mounted on the front wheel side or in theengine room, whereby the length of pipings from the pump 1 to the rearwheel suspensions 100fr, 100fL. Accordingly, a pressure drop caused bythe piping path is greater for the rear wheels, and if an oil leakageoccurs in the piping, the pressure drop will be greatest for the rearwheels. Accordingly, a line pressure detecting sensor 13rm is connectedto the rear wheel high pressure feed piping 9. On the other hand, thepressure of the reservoir return path 11 will be lowest at its endlocated nearer the pump 1, and will tend to increase in a direction awayfrom the reservoir 2. Accordingly, the pressure of the reservoir returnpath 11 is also detected by a pressure sensor 13rt which is locatedtoward the rear wheel.

The piping is connected to a bypass valve 120, which is effective toregulate pressure in the high pressure feed piping 8 to a valve whichcorresponds to a current level used to energize an electrical coil ofthe valve 120, thus obtaining a required line pressure. When an ignitionswitch is opened to stop the operation of the engine and hence the pump1, the line pressure is reduced substantially to zero or drained throughthe reservoir return path 11 to the atmospheric pressure which prevailsin the reservoir 2, thus reducing the loading upon the engine or pumpwhen restarting. The reduction in the line pressure turns the cut valves70fr, 70fL, 70rr, 70rL off, thus preventing a pressure release from theshock absorbers.

FIG. 2 shows a longitudinal section, to an enlarged scale, of thesuspension 100fr. A piston 103 is fixedly mounted on the piston rod102fr of the shock absorber 101fr, and extends through an inner sleeve104 to divide its interior into an upper chamber 105 and a lower chamber106. An oil pump pressure which represents the suspension sustainpressure is fed to the piston rod 102fr from the output port of the cutvalve 70fr, which pressure is passed through a side opening 107 in thepiston rod 102fr to be applied to the upper chamber 105 disposed insidethe inner sleeve 104, and thence passed through a verticalthrough-opening 108 formed in the piston rod 103 to be applied to lowerchamber 106. A sustain pressure which is proportional to the product ofthe pressure applied to the lower chamber 106 and cross-sectional area(the square of the rod radius multiplied by π) of the piston rod 102fris applied to the piston rod 102fr.

The lower chamber 106 communicates with a lower space 110 in anattenuator valve unit 109, which has an upper space that is divided by apiston 111 into a lower chamber 112 and an upper chamber 113. Oil fromthe lower space 110 passes through the valve unit 109 into the lowerchamber 112 while a high pressure gas is confines in the upper chamber113.

If the piston rod 102fr plunges rapidly downward into the inner sleeve104 as a result of a bumping reaction of the front, right wheel, theinternal pressure of the inner sleeve 104 increases rapidly, andsimilarly, the pressure in the lower space 110 tends to increase abovethe pressure in the lower pressure 112 rapidly. At this time, oil flowsfrom the lower space 110 into the lower chamber 112 through a checkvalve which permits a flow of the oil from the lower space 110 to thelower chamber 112 above a given pressure differential across the valveunit 109, but which blocks a passage of the oil in the oppositedirection, whereby the piston 111 moves up, thus buffering thetransmission of upward impact applied from the wheel to the piston rod102fr. In this manner, the transmission of bumping effect of the wheelto the car body is buffered.

If the piston rod 102fr tends to be withdrawn upwardly from within theinner sleeve 104 as the front, right wheel goes down rapidly, theinternal pressure of the inner sleeve 104 reduces rapidly, again causingthe pressure of the lower space 110 to be rapidly reduced below thepressure of the lower chamber 112. As this time, oil flows from thelower chamber 112 to the lower space 110 through a check valve whichpermits a flow of the oil from the lower chamber 112 to the lower space110 above a given pressure differential across the valve unit 109, butwhich blocks a passage of the oil in the opposite direction, whereby thepiston 111 moves down, buffering the transmission of downward impactapplied from the wheel to the piston rod 102fr. In this manner, thetransmission of the impact applied to the wheel or the falling effect ofthe car body is buffered.

As the pressure applied to the shock absorber 101fr increases in orderto an increase of the vehicle elevation, the pressure in the lowerchamber 112 rises, which in turn causes the piston 111 to be raised,allowing the latter to assume a position which corresponds to a loadingupon the car body.

When there is not vertical movement of the piston rod 102fr relative tothe inner sleeve 104 as during a parking condition, a seal between theinner sleeve 104 and the piston rod 102fr prevents any substantial oilleakage from the inner sleeve 104 to an outer sleeve 114. However, it isdesirable that such a seal will exhibit a sealing characteristic whichpermits a very limited amount of oil leakage during the verticalmovement of the piston rod 102fr in order to reduce a resistancepresented to such movement of the rod 102fr. Any oil leaking to theouter sleeve 114 is returned to the reservoir 2 through a drain 14fr(FIG. 1a) which is open to the atmosphere and a drain return piping 12(FIG. 1a), which represents a second return path. The reservoir 2 isequipped with a level sensor 28 (FIG. 1a), which produces an oilshortage signal when the oil level in the reservoir 2 falls below alower limit.

It should be understood that other suspensions 100fL, 100rr and 100rLare constructed in substantially the same manner as the suspension 100frmentioned above.

FIG. 3 shows a longitudinal section, to an enlarged scale, of thepressure control valve 80fr. It includes a sleeve 81, which is centrallyformed with a spool receiving opening, the inner surface of which isformed with an annular groove 83 communicating with a line pressure port82 and another annular groove 86 communicating with a low pressure port85. An output port 84 opens into the sleeve at a location in between theannular grooves 83, 86. A spool 90 is inserted into the spool receivingopening, and intermediate its length, its peripheral surface is formedwith an annular groove 91 having a width which corresponds to thedistance between a right edge of the groove 83 and a left edge of thegroove 86. The left end of the spool 90 is formed with a valve receivingopening, which communicates with the groove 91, and into which a valveelement 93 is inserted and held in place by a coiled compression spring92. The valve element 93 is centrally formed with an orifice extendingtherethrough, which provides communication between the space in whichthe groove 91 and hence the output port 94 is located and the space inwhich the valve element 93 and the spring 92 are received. Accordingly,at its left end, the spool 90 is subject to a pressure from the outputport 84 or a regulated pressure which is applied to the suspension100fr, whereby it is urged to the right. In the event pressure from theoutput port 84 increases in an impulse manner, the valve element 93 isdriven to the left against the resilience of the spring 92, creating abuffering space to the right of the valve element 93. Accordingly, whenthe pressure from the output port 94 rises in an impulse manner, thepressure rise is not immediately applied to the left end face of thespool 90, and thus the valve element 93 provides a buffering action upona movement of the spool 90 to the right in response to an impulse-likepressure rise, or pressure surge from the output port 84. Conversely, italso exerts a buffering action upon a movement of the spool 90 to theleft in response to an impulse-like pressure fall from the output port84.

The right end face of the spool 90 is subject to pressure from a targetpressure space 88 communicating with a high pressure port 87, whichpressure is supplied through an orifice 88f, whereby the spool 90 isurged to the left. A line pressure is fed to the high pressure port 87while the target pressure space 88 communicates with a low pressure port89 through a channel 94, the channel opening of which is determined by aneedle valve 95. When the needle valve 95 closes the channel 94, thetarget pressure space 88 which communicates with the high pressure port87 through the orifice 88f assumes the line pressure of the port 87,whereby the spool 90 is driven to the left, allowing the groove 91 inthe spool 90 to communicate with the groove 83 or the line pressure port82 thus causing the pressure of the port 91 or output port 84 to risewhich is then transmitted to the left of the valve element 93, thusimparting a rightward driving force to the left end of the spool 90.When the needle valve 95 is located to leave the channel 94 fully open,the pressure from the space 88 will be substantially reduced below theline pressure from the high pressure port 87 because of the restrictionpresented by the orifice 88f. Accordingly, the spool 90 moves to theright, allowing the groove 91 in the spool 90 to communicate with thegroove 86 or the low pressure port 85, reducing the pressure in the port91 or the output port 84. Such pressure is transmitted to the left ofthe valve element 93, thus reducing the rightward driving force which isapplied to the left end of the spool 90. In this manner, the spool 90assumes a position where the pressure from the target pressure which issubstantially proportional to the pressure from the target pressurespace 88 appears at the output port 84.

The pressure in the target pressure space 88 is determined by theposition of the needle valve 95, which is in turn substantially ininverse proportion to the distance between the needle valve 95 and thechannel 94, and hence there appears at the output port 84 a pressurewhich is substantially inversely proportional to the distance of theneedle valve 95.

A stationary core 96 of magnetizable material is shaped to allow theneedle valve 95 to extend therethrough, and has a right end which is inthe form of a truncated cone, which is opposed by a conical end face ofa plunger 97, also formed of a magnetizable material, which defines anopening having a closed bottom. The needle valve 95 is secured to theplunger 97. The core 96 and the plunger 97 extend into a bobbin whichcarries an electrical coil 99 thereon.

When the oil 99 is energized, there is established a loop for a magneticflux comprising the core 96, a yoke of magnetizable material 98a, an endplate of a magnetizable material 98b and the plunger 97 and then back tothe core 96. The plunger 97 is attracted toward the core 96 and thusmoves to the left, bringing the needle valve 95 closer to the channel 94or reducing the distance mentioned above. It will be noted that the leftend of the needle valve 95 is subject to pressure from the targetpressure space 88 which acts to drive it to the right while the rightend of the needle valve 95 is subject to the atmospheric pressurethrough a low pressure port 98c which is open to the atmosphere, so thatthe needle valve 95 will be driven to the right by the pressure from thetarget pressure space 88 with a force which depends on the magnitude ofsuch pressure (it will be noted that this corresponds to the position ofthe needle valve 95). As a consequence, the needle valve 95 is spacedfrom the channel 94 by a distance which is virtually inverselyproportional to the current level which is used to energize the coil 99.Either the core and the plunger is shaped as a truncated cone while theother is shaped as a complementary conical opening in order to achieve alinear relationship between the current level and distance.

As a consequence of such an arrangement, an output at the output port 84is substantially proportional to the current level used to energize thecoil 99. The pressure control valve 80fr delivers at the output port 94which is proportional to the current level within a given range.

When the coil 99 is energized with a current level for maintaining theheight of the vehicle to a desired level and when a wheel associatedwith the suspension 100fr rides on a hill on a bumpy road and rises up,the pressure of the suspension 100fr increases and consequently thepressure of the output port 84 of the pressure control valve 80frincreases to drive the spool 90 toward a pressure down direction (towardthe right in FIG. 3), substantially eliminating a transmission of an upimpact of the wheel to the body of the vehicle. The movement of thespool 90 toward the right causes an increase of the pressure in thetarget pressure space 88. The pressure in the target pressure space 88is applied to the top of the needle valve 95 through the channel 94.Thus the needle valve 95 goes back (moves toward the right) to have aspace from the channel 94. In other words, the communication from thetarget pressure space 88 to the return path 11 through the channel 94and the low pressure port 89 is increased. After the rising up of thewheel, the wheel falls down and consequently the pressure of thesuspension 100fr decreases, whereby the pressure of the output port 84of the pressure control valve 80fr decreases to drive the spool 90toward a pressure up direction (toward the left in FIG. 3),substantially eliminating a transmission of a down impact of the wheelto the body of the vehicle. The movement of the spool 90 toward the leftcauses a decrease of the pressure in the target pressure space 88,whereby the needle valve 95 goes forward (toward the left) to close thechannel 94 and consequently the communication of the target pressurespace 88 to the return path 11 decreases. Thus the pressure in thetarget pressure space 88 increases.

When the wheel falls down into a hollow of a bumpy road, the pressure inthe suspension 100fr decreases and consequently the spool 90 movestoward the left, substantially eliminating a transmission of a downimpact of the wheel to the body of the vehicle as mentioned above, andwhen the wheel rises up from the hollow of the bumpy road, the pressurein the suspension 100fr increases and consequently the spool 90 movestoward the right, substantially eliminating a transmission of an upimpact of the wheel to the body of the vehicle as mentioned above.

In the apparatus for supporting a car chassis as shown in FIGS. 1a and1b, the main check valve 50 supplies oil from the high pressure port 3to the high pressure piping 8, but blocks a reverse flow from the piping8 to the port 3. The relief valve 60m suppresses the pressure at thehigh pressure port 3 or the high pressure piping 8 at or below a givenhigh pressure, and in the event a high pressure surge is applied to theport 3, it releases such surge to the return path 11, thus buffering thetransmission of a pressure surge to the piping 8.

The bypass valve 120 controls the pressure of the rear wheel highpressure feed piping 9 substantially linearly in a given range, andmaintains the pressure of the piping 9 at a given value during asteady-state operation. The constant pressure control takes place bycontrolling the current level of the bypass valve 120 with reference toa pressure detected by the pressure sensor 13rm. In the event a highpressure surge is applied to the rear wheel suspension, the valvereleases it to the return path 11, thus buffering its transmission tothe piping 8. When the ignition switch is open, and the engine as wellas the pump 1 cease to operate, the energization is interrupted, wherebythe piping 9 is made to communicate with the return path 11, thusdecompressing.

Pressure control valves 80fr, 80fL, 80rr and 80rL deliver the requiredsustain pressure to output ports (84) by controlling the current levelof the electrical coil (99) so that the required sustain pressure isapplied to the suspension through the suspension pressure control. Thetransmission of a pressure surge to the suspension is buffered, thussuppressing a hunting of the pressure controlling spool (91), thusallowing the pressure applied to the suspension to be stabilized.

Cut valves 70fr, 70fL, 70rr and 70rL interrupt the suspension pressurefeeding line between the output port 84 of the pressure control valveand the suspension to prevent the pressure from being released from thesuspension when the line pressure (front wheel high pressure feed piping9) is below a given low pressure, and fully opens the feed line wheneverthe line pressure is equal to or greater than the given low pressure. Inthis manner, the suspension pressure is automatically prevented fromgoing abnormally low value when the line pressure is low.

Relief valves 60fr, 60fL, 60rr and 60rL limit the pressure of thesuspension feed line between the output port 84 of the pressure controlvalve and the suspension or principally the suspension pressure to lessthan an upper limit so that any high pressure surge applied to the feedline or the suspension as when the vehicle is bumped or a load of highmass is thrown onto the vehicle may be released to the return path, thusbuffering the impact upon the suspension and enhancing the durability ofthe feed line and its connected mechanical elements.

FIGS. 4a and 4b show an electrical control system which determines thedriving condition and the attitude of a vehicle in response to thestatus of various switches and sensors mounted on the vehicle andestablishes required pressures in the individual suspensions shown inFIGS. 1a and 1b so as to bring the attitude of the car body to a desiredone.

Vehicle elevation sensors 15fL, 15fr, 15rL, and 15rr are connected to alow pass filter 31a, which cuts off high frequency components or noisesin an analog signal which is detected by the respective sensors, andsmoothes a relatively high frequency component which is shaped toprovide a vehicle elevation signal, which is then amplifies to reach agiven range of levels by an amplifier 30a before it is applied to A/Dconverter (IC) 29a.

Pressure sensors 13fL, 13fr, 13rL and 13rr which detect the oil pressureof the individual suspensions are connected to a low pass filter 31b,which cuts off high frequency components or noises from an analog signalrepresenting a pressure detected by individual pressure sensors andsmoothes a relatively high frequency component, which is then shaped toprovide a pressure signal, that is in turn amplified to reach a givenrange of levels by an amplifier 30b before it is applied to A/Dconverter (IC) 29b.

Pressure sensor 13rm which detects the pressure of the rear wheel highpressure feed piping 9 and pressure sensor 13rt which detects thepressure of the rear wheel side of the return path 11 are connected to alow pass filter 31c, which cuts off high frequency components or noisesfrom an analog signal representing a pressure detected by the respectivepressure sensors and smoothes a relatively high frequency component toprovide a shapes pressure signal, which is then amplified to reach agiven range of levels by an amplifier 30c before it is applied to A/Dconverter (IC) 29c.

A longitudinal acceleration sensor 16p which detects the acceleration inthe fore-and-aft direction of the vehicle (a positive value representingan acceleration and a negative value representing acceleration) and alateral acceleration sensor 16r which detects the acceleration in thelateral direction of a vehicle (a positive value representing anacceleration directed from left to right and a negative valuerepresenting an acceleration directed from right to left) are alsoconnected to the low pass filter 31c, which cuts off high frequencycomponents or noises from an analog signal representing a pressuredetected by the respective acceleration sensor and smoothes a relativelyhigh frequency component to provide a shaped acceleration signal, whichis then amplified to reach a given range of levels by an amplifier 30cbefore it is applied to A/D converter (IC) 29c.

The electrical coils 99 of the pressure control valves 80fL, 80fr, 80rLand 80rr as well as the electrical coil 129 of the bypass valve 120 areconnected to coil drivers 33. Each driver 33 comprises a switchingcircuit which is operative to energize individual electrical coil, and acurrent detector which detects the current level of the respective coiland produce an analog signal representing same. A duty controller (IC)32 provides an ON (energization)/OFF (deenergization) command. Inresponse to the on command, the driver 33 completes the connectionbetween a selected coil and the output of a constant current circuit.The OFF command causes such connection to be disconnected. Analogvoltages representing the current levels detected are normally fed toA/D converter (IC) 29c.

The duty controller 32 receives data representing the current level tobe used in the energization for each of the electrical coils (which areassociated with the pressure control valves and the bypass valve) andwhich is supplied from a microprocessor (hereafter abbreviated as CPU)18 and stores it in a latch, feeds a detected current level to the CPU18 through A/D converter (IC) 29c for feedback purpose, controls theduty cycle so as to achieve the current level specified by the CPU 18,and deliver a time sequence of ON/OFF commands which correspond to theduty cycle to the coil driver 33.

Each of the A/D converters 29a to 29c is formed by an integrated circuitinternally housing a sample-and-hold circuit having four input ports,except for the converter 29c which is fed with analog voltages from thecoil driver 33, representing detected current levels of the pressurecontrol valve and the bypass valve. In response to a conversion commandfrom the CPU 18, the converter samples an analog voltage at its inputport in its sample-and-hold circuit and converts it to digital datawhich may be vehicle elevation, pressure or acceleration data, which isthen transferred to the CPU 18 serially in synchronism with a clockpulse fed from the CPU 18. The sampling, the conversion and the transferoperation take place successively with respect to the input ports. Inthis manner, a single conversion command from the CPU 18 is effective tocause the converter to sample four analog voltages at its input portssuccessively and to transfer resulting digital data serially to the CPU18.

The CPU 18 is arranged to communicate data with the CPU 17, whichreceives a number of signals including an open (L:no pressure controlcommand)/close (H:command) signal from a command switch CSW whichcommands a pressure control of the suspension; a signal from a brakepedal having H (depressed)/L (no depression) level; an open (L)/close(H) signal from the ignition switch 20; pulses from a vehicle speedsynchronized pulse generator 25 which generates one pulse perincremental angle of rotation of an output shaft of an onboardtransmission; a first set and a second set of pulses from a rotaryencoder 26 which produce one pulse per incremental angle of rotation ofa steering shaft, the pulses in the second set being 90° phase displacedfrom the pulses in the first set; data generated by an absolute encoder27 coupled to the rotary shaft of a throttle valve of the engine andproducing 3-bit data representing the opening of the throttle valve; anda signal from a level sensor 28 which detects the oil level in thereservoir 2 (H:below a lower limit level), L:a level higher than thelower limit level). In addition, signals from other sensors, not shown,are fed to the CPU 17 through an input/output circuit 34. An indicator,such as a warning light, is also connected to the input/output circuit34, and is operated by the CPU 17 through the input/output circuit 34 inthe event the occurrence of any abnormality in the pressure control ofthe suspension is determined.

An onboard battery 19 is connected to a backup power supply circuit 23having a reduced capacity and which feeds a constant voltage to the CPU17. Accordingly, as long as the battery 19 provides an output voltagewhich is at or above a given value, the CPU 17 is normally maintainedoperative to preserve data in its internal memories.

The onboard battery 19 is also connected through the ignition switch 20to a constant voltage power supply circuit 21 having an increasedcapacity, which delivers constant voltages of low level to electronicelements and circuits such as the CPU 18 and also delivers constantvoltages of high levels to selected circuits such as low pass filters31a to 31c and input/output circuit 34. The ignition switch 20 isshunted by a self holding contact of a relay 22, which may be turned ONand OFF by the CPU 17.

Each of the CPU's 17 and 18 has respective programs stored therein whichcontrols the pressures of the suspensions. The CPU 18 operates accordingto such program, principally operating to read values detected by thevehicle elevation sensors 15fL, 15fr, 15rL, 15rr, the pressure sensors13fL, 13fr, 13rL, 13rr, 13rt and the onboard longitudinal and lateralacceleration sensors 16t, 16r and to control the current level which isused to energize the electrical coils 99, 129 of the pressure controlvalves 80fL, 80fr, 80rr and the bypass valve 120.

By contrast, the CPU 17 operates to establish or terminate the linepressure for the suspension system (FIGS. 1a and 1b), to determine thedriving condition of the vehicle, and to calculate pressures required ofthe suspension in order to establish a vehicle elevation and vehicleattitude which are appropriate to the result of the determination duringan interval from the turn-ON to the turn-OFF of the ignition switch 20as well as during a short time internal thereafter. At this end, the CPU17 receives various detected values from the CPU 18 in order todetermine the driving condition of a vehicle, and delivers the currentlevel required for the energization to establish the required pressureto the CPU 18.

Referring to FIG. 5a and subsequent figures, the control operationperformed by the CPU 17 and the CPU 18 will now be described. It is tobe understood that in the description to follow as will as in the flowcharts shown in the drawings, the nomenclature used to designate theregister itself may also indicate the content which is stored in theregister.

Initially referring to FIG. 5a, when the power supply is turned on (instep S1 where the backup power supply circuit 23 is activated to produceconstant voltages or the battery 19 is mounted on the vehicle), the CPU17 initializes its internal registers, counters and timers, and deliversignal levels at its output ports which establish initial standbyconditions (this involves no energization of mechanical elements) (instep 2). The CPU 17 then examines if the command switch CSW is closed(in step S3: more precisely, if the command switch CSW and the ignitionswitch 20 are closed), and if it remains open, it waits for the switchto be closed.

When the command switch CSW is closed, the coil of the relay 22 isenergized to close the self-holding contacts (in step S4). As a resultof a closure of the ignition switch 20, the power supply circuit 21having an increased capacity is connected to the battery 19, andoperates to deliver constant voltages of low level to electroniccomponents such as the CPU 18 and other electrical circuits and todeliver constant voltages of high level to low pass filters 31a to 31cand input/output circuit 34. In this manner, the CPU 18 is alsoelectrically activated to be operative. Since the relay contacts areclosed to maintain the power supply circuit 21 connected to the battery19, the electrical circuit system shown in FIGS. 4a and 4b are entirelyelectrically activated to remain operative if the ignition switch 20 issubsequently opened unless the relay 22 is turned OFF by the CPU 17.

After energizing the relay coil, the CPU 17 enables an interruptoperation which is executed in response to an oncoming pulse signal toits interrupt input ports ASRO to ASR2 (in step S5).

The interrupt operation will be initially described. Considering aninterrupt operation which takes place in response to a pulse generatedby the vehicle speed synchronized pulse generator 25 (applied to inputport ASR2), as the generator 25 generates a single pulse, the CPU 17advances to an interrupt operation (ASR2) where the content of a vehiclespeed measuring register is read and the register is re-started. Avehicle speed is calculated on the basis of the content read(representing the time period for the pulses), a weighted mean Vs isderived over previous several values of the vehicle speed calculatedwhich have been stored and the weighted mean written into a vehiclespeed register VS, and then the operation returns to the main program.As a result of the execution of the interrupt operation (ASR2), data Vsrepresenting the prevailing vehicle speed or a value which is smoothedover previous calculated values in the time sequence is alwaysmaintained in the vehicle speed register VS.

When a first set of pulses are produced by the rotary encoder 26 whichdetects the direction of rotation of the steering shaft to initiate theinterrupt operation (input port ASRO), the interrupt operation (ASRO) isentered in response to a rising and a falling edge of the pulses in thefirst set. When the interrupt operation (ASRO) is entered in response tothe rising edge, "H" is written into a flag register which is used todetermine the direction of the register. When the interrupt operation(ASRO) is entered in response to the falling edge, the flat register iscleared (or "L" is entered), subsequently returning to the main program.

It will be seen that if a rising edge of a pulse in the first set fromthe rotary encoder 26 (flat register=H), this means that the steeringshaft is driven to rotate counter-clockwise. On the contrary, if thefalling edge of a pulse in the first set (flag register=L), is followedby the rising edge of a pulse of the second set, the steering shaft isdriven for clockwise rotation.

When an interrupt operation (input port ASR1) is entered in response toa second set of pulses generated by the rotary encoder 26 which detectsthe rotational speed (steering angular velocity) of the steering shaft,a pulse (or its falling edge) in the second set initiates the interruptoperation (ASR1) wherein the content of a steering measuring register isread and the register re-started, a positive (counter-clockwiserotation) or a negative (clockwise rotation) sign is applied to the readcontent which indicates the time period of pulses generated insynchronism with the steering angular velocity when the content of theflag register is "H" or "L", respectively, and a speed value inclusiveof the direction sign is calculated therefrom, a weighted mean Ss overprevious several values calculated in derived and written into asteering angular velocity register Ss, and then the operation returns tothe main program. As a result of the execution of the interruptoperation (ASR1), data Ss (where a positive value represents acounter-clockwise rotation while a negative value indicates a clockwiserotation) representing the prevailing steering angular velocity registerSS.

When the CPU 17 has enabled the interrupt operation, it examines if theCPU 18 issues a ready signal (in step S6). When the power supply to theCPU 18 is turned ON as when the ignition switch 20 is closed or thepower supply circuit 21 delivers an output of Vc=5V, the CPU 18initializes its internal registers, counters and timers, and delivers atits output ports signal levels for establishing initial standbyconditions which involves no electrical energization of mechanicalelements. Data are supplied to the duty controller which designate allof the electrical coils to be turned off. The CPU 18 then delivers amaximum current level data which causes the full closure of the bypassvalve 120, and then commands the energization of the bypass valve 120.As a result of the above arrangement, the current level for all of thepressure control valves 80fL, 80fr, 80rL and 80rr are equal to zero, anda pressure equivalent to that prevailing in the return path 11 isdelivered to its output port (84). Since the bypass valve 120 is in fullclosure and since the ignition switch 20 is closed and the pump 1 isbeing drive for rotation, the pressure in the high pressure feed piping8, the front wheel high pressure feed piping 6 (accumulator 7) and therear wheel high pressure feed piping 9 (accumulator 10) begins to rise.Subsequently, during a first period, the CPU 18 reads values detected bythe vehicle elevation sensors 15fL, 15fr, 15rL, 15rr, the pressuresensors 13fL, 13fr, 13rL, 13rr, 13rm, 13rt, the longitudinalacceleration sensor 16p and the lateral acceleration sensor 16r as wellas the current levels detected from the coil drivers 33, all of whichare used to update its internal registers. When the CPU 17 delivers datarepresenting the current levels of the pressure control values 80fL,80fr, 80rL, 80rr and the bypass valve 120, the CPU 18 transfers it tothe duty controller 32.

If the CPU 17 finds at step S6 that the CPU 18 delivers a busy signal,it loops around standby loop (in steps S8 to S11). If the CPU 18delivers a ready signal, the standby looping is terminated (in stepS13).

The CPU 17 requests the CPU 18 to transfer detected pressure data Dphfrom the pressure sensor 13rm, and then receives it and writes it intoregister DPH (in step S14), and then examines if the detected pressureDph representing the rear wheel side pressure of the high pressure feedpiping 8 has become equal to or greater than a given value Pph, which isless than a given low pressure at which the cut valves 70fL, 70fr, 70rL,70rr begin to be opened or if the line pressure has risen to a degree(in step S15). If the line pressure has not risen, the operation returnsto step S6.

Referring to FIG. 5b, when the line pressure has risen (up to Phs), theCPU 17 commands the CPu 18 to transfer data PfLo, Pfro, PrLo, Prrorepresenting the initial pressures detected by the pressure sensors13fL, 13fr, 13rL, 13rr, which are then written into registers PFLo,PFRo, PRLo, PRRo upon receipt (in step S16).

The content PfLo, Pfro, PrLo, Prro of the registers PFLo, PFRo, PRLo,PRRo are used to access data representing the current level which isrequired to achieve a required pressures and which are stored in a givenregion (Table 1a) of the internal ROM, thus the reading from the Table1a the current level IhfL which is required to the coil 99 to deliverthe pressure PfLo to the output port 84 of the pressure control valve80fL; the current level Ihfr which is required to deliver the pressurePfro to the output port of the pressure control valve 80fr; the currentlevel IhrL required to deliver the pressure PrLo to the output port ofthe pressure control valve 80rL and the current level Ihrr required todeliver the pressure Prro to the output port of the pressure controlvalve 80rr, all of which are written into output registers IHfL, IHfr,IHrL and IHrr (in step S17). Data in these output registers aretransferred to the CPU 18, which delivers them to the duty controller 32upon receipt.

The duty controller 32 stores data representing the current levels IhfL,Ihfr, IhrL and Ihrr in respective latches and then regulates the ON/OFFduty of the coil 99 associated with the pressure control valve 80fL sothat the current level which is fed back from the pressure control valve80fL through the CPU 18 becomes equal to IhfL. A time sequence of ON/OFFcommands which corresponds to such duty is applied to the coil driver 33so as to control the pressure control valve 80fL. The duty control ofthe remaining pressure control valves 80fr, 80eL, 80rr takes place in asimilar manner, applying a time sequence of ON/OFF commands to the coildriver 33. When the current levels are established in this manner, thepressure control valves 80fL, 80fr, 80rL 80rr deliver the pressureswhich are substantially equal to PfLo, Pfro, PrLo, Prro, to the outputport (84) if the line pressure is equal to or greater than a given lowpressure. When the cut valves 70fL, 70fr, 70rL, 70rr are opened inresponse to the line pressure rising to or above the given low pressure,pressures which are substantially equal to the initial pressure PfLo,Pfro, PrLo, Prro of the individual suspensions are supplied through thecut valves 70fL, 70fr, 70rL, 70rr and the pressure control valves 80fL,80fr, 80rL 80rr to the suspensions 100fL, 100fr, 100rL, 100rr,respectively. Accordingly, when the command switch CSW is closed and theignition switch 20 changes from its open (the engine and the pump 1 notoperating) to its closed (the pump 1 being driven) condition and the cutvalves 70fL, 70fr, 70rL, 70rr are open to achieve the line pressurewhich is equal to or greater than the given low pressure and the oilpressure lines of the suspension communicate with the output ports ofthe pressure control valve or when the ignition switch 20 is closed andthe command switch CSW changes from its open condition to its closedcondition, the pressures delivered by the pressure control valves willbe substantially equal to the suspension pressures, preventing a rapidpressure variation in the suspensions. In other words, an impulse-likechange in the attitude of the vehicle is avoided.

The above description covers the establishment of initial outputpressures from the pressure control valves 80fL, 80fr, 80rL, 80rr whenthe command switch CSW is closed and the ignition switch 20 is changedfrom its open to its closed condition or immediately after the enginehas started. Also the above description covers the establishment ofinitial output pressures from the pressure control valves 80fL, 80fr,80rL, 80rr when the ignition switch 20 is closed and the command switchCSW is changed from its open to its closed condition.

Subsequently, the CPU 17 starts timer ST which provides a time limit (instep S19). The content of the register ST is represented by ST, and dataST representing a second time period which is longer than the first timeperiod during which the CPU 18 reads detected values is written into theregister ST.

Upon starting the timer ST, the CPU 17 reads the status (in step S20).Specifically, the open/close signal of switch BTS which detects thedepression of the brake pedal, data representing the throttle openingfrom the absolute encoder 27, and a signal from the reservoir leveldetecting switch 28 are read and written into internal registers. At thesame time, the CPU 17 commands the CPU 18 to transfer detected data,whereby data DfL, Dfr, DrL, Drr representing the vehicle elevationsdetected by the sensors 15fL, 15fr, 15rL, 15rr, data PfL, Pfr, PrL, Prr,Prm, Prt representing the pressures detected by the sensors 13fL, 13fr,13rL, 13rr, 13rm, 13rt, and data representing the detected current levelof the pressure control valves 80fL, 80fr, 80rL, 80rr and the bypassvalve 120 are transferred to the CPU 17, which then write them intointernal registers. A reference to these read value is made to render adecision concerning the abnormality/normality, and in the event anabnormality is found, the program proceeds to step S8.

In the event a normality is found, the CPU 17 then executes a linepressure control (LPC). Specifically, it derives the absolute value andthe polarity (high/low) of a deviation of a detected line pressure Prmwith respect to a reference pressure which is a fixed value slightlyless than a relief pressure (which is referred to as a given highpressure) of the relief valve 60m, and adds a correction value whichreduces the deviation to zero to the current level that is currently fedto the bypass valve 120 to derive a new current level for the bypassvalve 120, which is then written into an output register. The content ofthis output register is transferred to the CPU 18 at a subsequent stepS36. As a result of this line pressure control (LPC), the current levelof the bypass valve 120 will be controlled so that the pressure of therear wheel high pressure feed piping 9 assumes a given value slightlyless than the relief pressure (the given high pressure) of the reliefvalve 60m.

Referring to FIG. 5c, upon completion of the line pressure control (LPC)the CPU 17 examines the switch CSW to see if it is open or closed (instep S21A), and if it is open, it executes a stop operation (in stepS23) where the relay 22 is deenergized and the interrupt operations(ASR0 to ASR2) are inhibited. In the stop operation (in step S23), thebypass valve 120 is initially deenergized so that it becomes fully open,thus releasing the line pressure to the return path 11. When the switch20 is open and hence the engine as well as the pump 1 cease to operate,and the discharge of a high pressure from the pump 1 ceases, or when thecommand switch CSW is open even if the ignition switch 20 is closed, thebypass valve 120 which is now fully open allows the pressures in thehigh pressure feed piping 8, the front wheel high pressure feed piping 6(accumulator 7) and the rear wheel high pressure feed piping 9(accumulator 10) to be reduced to the pressure of the return path 11,which is in turn released to the reservoir 2. Thus all of the pipingsassume an atmospheric pressure. At a timing when all of the pipingsreduces to or below the given low pressure at which the cut valves 70fL,70fr, 70rL, 70rr are fully closed, the CPU 17 operates to deenergize thepressure control valves 80fL, 80fr, 80rL, 80rr.

When the switches 20 and CSW are closed, the CPU 17 examines if "test"is indicated or not, and if "test" is indicated the CPU 17 examines ifthe vehicle speed Vs is lower than a predetermined value A or not (steps22B to 22D). Namely the CPU 17 examines if test indication switches TWS1and TSW2 are closed (in steps S22b and S22B), and if they are closed theCPU 17 examines if the vehicle speed Vs is lower than A or not (in stepS22D).

If the switches TSW1 and TSW2 are closed and the vehicle speed Vs islower than A, the CPU 17 executes a subroutine "test" (CTS), the contentof which is shown in FIGS. 6h and 6i and described later. After theexecution of "test" (CTS), the CPU 17 executes "output" (subroutine 36)in which the CPU 17 supplies the CPU 18 with pressure indication datacalculated in the "test" subroutine CTS and assigned to the pressurecontrol valves 80fL, 80fr, 80rL, 80rr.

If "test" is not indicated, the CPU 17 calculates a parameterrepresenting the running condition of the vehicle (in step S25). ThenCPU 17 executes "calculation of vehicle elevation deviation" (in stepS31) where a deviation of the actual vehicle elevation with respect to atarget elevation is calculated, and a correction to be applied to thesuspension pressures, hereafter referred to as a first correction foreach of the suspensions, is calculated which reduces the deviation tozero. The detail of this operation will be described later in connectionwith FIGS. 6a and 6b.

"Calculation of vehicle elevation deviation" (in step S31) is followedby "predictive calculation of pitching/rolling" (in step S32) where acorrection to the suspension pressure (which is hereafter referred to asecond correction for each of the suspensions), is calculated inaccordance with longitudinal and lateral accelerations which the vehicleactually experiences, the deriving an interim value "initial suspensionpressure (PfLo, Pfro, PrLo, Prro)+first correction+second correction".The detail of this operation will be described later in connection withFIGS. 6c and 6d.

The CPU 17 then executes "pressure correction" (in step S33) where the"interim value" mentioned above is again corrected in accordance withthe line pressure (high pressure) detected by the pressure sensor 13rmand the return pressure (low pressure) detected by the pressure sensor13rt. The detail of this operation will be described later in connectionwith FIG. 6e.

The CPU 17 then effects "pressure/current conversion" (in step S34)where the corrected "interim value" for each of the suspensions isconverted into the current level which is to be used in energizing thepressure control valves (80fL, 80fr, 80rL, 80rr). The detail of thisoperation will be described later in connection with FIG. 6f.

The CPU 17 then effects "correction of warp" (in step S35) where aturning warp correction, representing a correction to be applied to thecurrent level, which depends on the lateral acceleration Rg and thesteering rate Ss, is calculated and is added to the current level to beapplied to the pressure control valve. The detail of this operation willbe described later in connection with FIG. 6g.

The CPU 17 then comes to "output" (in step S36) where it delivers thecurrent levels to be applied to the respective pressure control valveswhich are calculated in the manner mentioned above to the CPU 18 so asto be fed to the individual pressure control valves, and also deliverthe current level which is to be applied to the bypass valve 120 ascalculated by the "line pressure control" (LPC) to the CPU 18 so as tobe fed to the bypass valve 120.

At this point, the CPU 17 has completed all the tasks which arecontained in one cycle of the suspension pressure control. The CPU 17then waits for the timer ST to time out (in step S37), whereupon itreturns to step 19, re-starting the timer ST and initiating theexecution of tasks in the suspension pressure control of next cycle.

In controlling the suspension pressures as described above, the CPU 17demands the CPU 18 to effect a transfer of detected values from thesensors with ST period (second period) (subroutine 20). In responsethereto, the CPU 18 transfers sensor data to the CPU 17, which is asmoothed version of a weighted mean of several past values which areread during the first period. Data representing the current levels whichare to be applied to the respective pressure control valves and thebypass valves 120 are transferred from the CPU 17 to the CPU 18 with theST period. In response to each transfer, the CPU 18 delivers suchcurrent level data to the duty controller 32, where they are latched.Accordingly, the duty controller 32 controls the duty cycle of theenergization so that the actual current levels of the pressure controlvalves and the bypass valve 120, as detected by the coil drivers 33,coincide with the target current levels while updating the targetcurrent levels themselves with the ST period.

Referring to FIGS. 6a and 6b, "calculation of vehicle elevationdeviation" as represented by step S31 will be described. To give ageneral summary initially, data DfL, Dfr, DrL, Drr, which are contentsof the registers DFL, DFR, DRL, DRR, representing vehicle elevationsdetected by the sensors 15fL, 15fr, 15rL, 15rr are used to determine anoverall vehicle heave (height) DHT, a pitch DPT representing adifference in the vehicle elevation between the front wheel and the rearwheel, a roll DRT representing a difference in the vehicle elevationbetween the right wheel and the left wheel, a warp DWT representing adifference between the sum of the vehicle elevations of the front, rightwheel and the rear, left wheel and the sum of the vehicle elevations ofthe front, left wheel and the rear, right wheel. Specifically, thevehicle elevations of the respective wheels as represented by thecontent of the registers DFL, DRF, DRL, DRR are converted into a heaveDHT, a pitch DPT, a roll DRT, a warp DWT, which are attitude parametersof the overall vehicle.

DHT=DFL+DFR+DRL+DRR

DPT=-(DLF-DFR)+(DRL+DRR)

DRT=(DFL-DFR)+(DRL-DRR)

DWT=(DFL-DFR)-(DRL-DRR)

The calculation of the parameter DHT is executed in "calculation ofheave error CH" (in subroutine S50), the calculation of the parameterDPT is executed in "calculation of pitching error CP" (in subroutineS51), the calculation of the parameter DRT is executed in "calculationof rolling error CR" (in subroutine S52), and the calculation of theparameter DWT is executed in "calculation of warp error CW" (insubroutine S53).

In the "calculation of heave error CH" (in subroutine S50), a targetheave Ht is derived from the vehicle speed Vs and an indication of"normal" or "high" which is determined by a closure state of a heightindication switch HSW by accessing an internal ROM (table 2H) of the CPU17 with "normal" or "high" and Vs, the target heave Ht is written in theheave target register HT, and a heave error of the calculated heave DHTwith respect to the target heave Ht is calculated. For the purpose ofPID (proportional, integral and differential) control, the calculatedheave error is passed through PID processing, which derives a heavecorrection CH corresponding to the particular heave error.

Similarly, in the "calculation of the pitching error CR" (in subroutineS51), a target pitch Pt is derived from the longitudinal accelerationPg, and the pitch error of the calculated pitch DPT with respect to thetarget pitch Pt is calculated, and the calculated pitch error is subjectto PID processing, thus deriving a pitch correction CP which depends onthe particular pitch error.

Also, in the "calculation of rolling error CR" (in subroutine S52), atarget roll Rt is derived from the lateral acceleration Rg, and a rollerror of the calculated roll DRT with respect to the target roll Rt iscalculated, which is then subjects to PID processing, thus deriving aroll correction CR which depends on the particular roll error.

Finally, in the "calculation of warp error CW" (in subroutine S53), atarget warp Wt is initially assumed to be zero, and the warp error ofthe calculated warp DWT with respect to the target warp Wt iscalculated, and is then subject to PID processing, thus deriving a warpcorrection CW which depends on the particular warp error. When thecalculated warp error, which is equivalent to DWT since the target warpis assumed to be zero, has an absolute value which is less than a givenvalue representing a permissible range, the warp error which is subjectto PID processing is made equal to zero. When the error exceeds thegiven value, the warp error which is subject to PID processing ischanged to -DWT.

Describing the "calculation of the heave error CH" (in subroutine S50)in detail, the CPU 17 initially reads a target heave Ht corresponding tothe vehicle speed Vs from a region (table 2H) of the internal ROM andwrites it into a heave target register Ht (in step S39).

As indicated by "table 2H" in FIG. 6a, the target heave Ht which isgiven in correspondence to the vehicle speed Vs assumes a high value Ht1at low vehicle speed Vs equal to or less than 80 Km/h, and a low valueHt2 for high vehicle speed Vs equal to or greater than 120 Km/h, thetarget changes linearly with respect to the vehicle speed Vs, althoughit may follow a curve. The purpose of linearly changing the target is toprevent a degradation in the stability of the vehicle elevation as aresult of a frequent change of the vehicle speed at high speeds, whichwould be experienced if Vs is around 100 Km/h, as a result of a stepwisevariation in the target heave in response to a slight change in Vs whena target value of Ht1 is used for Vs equal to or less than 100 Km/h,while a target value Ht2 is used for Vs which is equal to or above 100Km/h. As utilizing the setting as shown in the table 2H, any slightchange in the vehicle speed Vs will only result in a slight change inthe target value, thus enhancing the stability of the vehicle elevation.

The CPU 17 then calculates the heave DHT (in step S40). The content ofthe register EHT2 in which the previously calculated heave error iswritten is then rewritten into register EHT1 (in step S41), and theheave error of the present pass HT-DHT is calculated, and is writteninto register EHT2 (in step S42). As a consequence, the register EHT1stores the heave error of the previous pass while the register EHT2stores the current heave error. The CPU 17 then writes the content ofregister ITH2 in which an error integral up to the previous pass isstored into register ITH1 (in step S43), and the current PID correctionITh is calculated as follows:

    ITh=Kh1·EHT2+Kh2 (EHTs+Kh3·ITH1)+Kh4·Kh5 (EHT2-EHT1)

where Kh1·EHT2 represents the proportional term in the PID calculation,Kh1 represents a factor of proportionality and EHT2 represents thecontent of register EHT2 or the current heave error. Kh2·(EHT2+Kh3 ITh1)represents I (integral) term, Kh2 represents an integrating coefficient,and ITH1 represents a correction integral up to the previous pass or anintegral of a correction output since the steps S16 to S18 where theinitial pressure is established. Kh3 represents a weighting coefficientwhich relates to the current error EHT2 and the correction integral ITH1together. Kh4·Kh5·(EHT2-EHT1) represents a D (differential) term havinga coefficient Kh4·Kh5, where Kh4 has a value corresponding to thevehicle speed Vs and Kh5 has a value corresponding to the steeringangular velocity Ss. Thus, a vehicle speed correcting coefficient Kh4which corresponds to the prevailing vehicle speed Vs is read from aregion (table 3H) of the internal ROM, and a steering angular velocitycorrecting coefficient Kh5 which corresponds to the prevailing steeringangular velocity Vs is read from a region (table 4H) of the internalROM, deriving their products Kh4·Kh5 as the coefficient for thedifferential term.

As illustrated by "table 3H" in FIG. 6a, the vehicle speed correctingcoefficient Kh4 may be regarded as assuming a higher value for a highervehicle speed Vs, thus increasing the significance of the differentialterm. It will be seen that the differential term provides a correctionwhich is effective to converge as rapidly to a target value as possiblein response to a change in the heave. This would be accomplished byincreasing the value of the coefficient in proportion to the vehiclespeed inasmuch as a change in the vehicle elevation occurs rapidly inresponse to disturbances when the vehicle speed is higher. On the otherhand, when the vehicle speed Vs exceeds a certain value which is chosento be 40 Km/h in the table 3H, it will be seen that a depression andrelease of a brake pedal, an operation of an accelerator pedal, aturning of a steering wheel to effect a turning or a turn-backoperation, if allowed to occur rapidly, will result in a rapid andexcessive change in the attitude of the car body. Accordingly, anydifferent term which provides a rapid compensation for such a rapidchange in the attitude will degrade the stability of controlling thevehicle elevation. For this reason, the vehicle speed correctingcoefficient Kh4 in the table 3H is designed to allow a greater changefor a low vehicle speed Vs and to allow a smaller change for a highervehicle speed Vs. In this manner, the significance of the differentialterm changes greatly in response to a change in the vehicle speed whenthe vehicle Vs is low, but a change which occurs in the significance ofthe differential term in response to a change in the vehicle speed willbe reduces when the vehicle speed Vs is high.

As shown at "table 4H" in FIG. 6a, the steering angular velocitycorrecting coefficient Kh5 can be summarized as exhibiting a greatervalue for a higher steering angular velocity Ss, thus increasing thesignificance of the differential term. This means that the differentialterm provides a correction which is designed so that a convergencetoward the target value takes place rapidly in response to a change inthe heave, and since the rate of change in the vehicle elevation inresponse to disturbances increases as the steering angular velocity Ssis higher, the differential term is designed to be enhanced inaccordance with the steering angular velocity. On the other hand, for asteering angular velocity Ss equal to or less than a certain value,which is chosen to be 50°/msec in the table 4H, a change in thetravelling direction takes place very slowly, and accordingly thesignificance of the differential term is chosen small. For a velocity inexcess of 5020 /msec and not exceeding 400°/msec, a change in thevehicle elevation takes place at a rate which is substantiallyproportional to the steering angular velocity Ss. At or above an angularvelocity of 400°/msec, a change in the attitude of a vehicle will occurvery rapidly and excessively, and any differential term which provides arapid and excessive compensation for such rapid change in the attitudewill degrade the stability of controlling the vehicle elevation. Forthis reason, the coefficient Kh5 for the differential term is chosen tobe a constant value for Ss equal to or less than 50°/msec, and is maderelatively high in substantial proportion to Ss when the latter exceeds50°/msec until 400°/msec is reached, and again assumes a constant valueat or above 400°/msec.

The introduction of the differential term Kh4·Kh5· (EHT2-EHT1) and thechoice of the coefficient Kh4 which increases with the vehicle speed Vsand of the coefficient Kh5 which increases with the steering angularvelocity Ss achieve a differential control over the weighting inaccordance with the vehicle speed Vs and the steering angular velocitySs to accomplish a high stability control over the vehicle elevation inresponse to a change in the vehicle speed Vs and the steering angularvelocity Ss.

When the heave error correction ITh is calculated in a PID calculationsubroutine (in step S44), the CPU 17 then writes the calculated heaveerror correction ITh into register ITH2 (in step S45), and multiplies itby a weighting coefficient Kh6, representing a weighting relative to apitch error correction, a roll error correction, and a warp errorcorrection, which will be described later, or stated differently, aspecific contribution in the total correction, and writes the resultinto heave error register CH.

After the execution of the subroutine S50 to calculate the heave errorCH, the CPU 17 executes the subroutine S51 to calculate the pitchingerror CP. The pitch error correction CP is calculated in the similarmanner as the calculation of the heave error CH, and the CPU 17 writesit into pitch error register CP. A pitch target value PT whichcorresponds to a heave target value HT is obtained by reading data Pt,representing a target value depending on the longitudinal accelerationPg, from one region (table 2P) of the internal ROM.

FIG. 7a shows the content of the table 2P. The pitch target value Ptacts to cancel the pitch which would appear as a result of thelongitudinal acceleration Pg. The purpose of a region a is to increasethe target pitch as the longitudinal acceleration Pg increases, therebyachieving a power savings. The purpose of a region b to prevent anyabnormality of the sensor in response to an abnormal value of Pg fromproviding a pitch target value even though there is no Pg developed inactuality, by reducing the pitch target value. In other respects, theoperation which occurs in this subroutine is similar to the subroutineS50 to calculate the heave error CH. Specifically, HT,Ht in step S39 maybe replaced by PT, Pt; the equation to calculate DHT at step S40 may bereplaced by the foregoing equation to calculate DPT; EHT1, EHT2appearing in step S41 may be replaced by EPT1, EPT2; EHT2, HT, DHTappearing in step S42 may be replaced by EPT2, PT, DPT; ITH1, ITH2appearing in step S43 may be replaced by ITP1, ITP2; the equation tocalculate ITh in the subroutine S44 may be replaced by a correspondingequation to calculate the pitch error correction ITp; table 3H may bereplaced by a coefficient table (4P) which is used to calculate thepitch correction ITp; ITH2, ITh appearing in step S45 may be replaced byITP2, ITp; and CH, Kh6, ITh appearing in step S46 may be replaced by CP,Kp6, ITp. In this manner, a flow chart showing the subroutine S51 tocalculate the pitch error CP in detail can be obtained. The CPU 17executes the processing operation represented by such flow chart.

Then the CPU 17 executes the subroutine 52 to calculate the rollingerror CR, and the rolling correction CR is calculated in the similarmanner as the heave error CH, and is then written into rolling errorregister CR. In this instance, a roll target value RT corresponding tothe heave target value HT may be obtained by reading data Rt whichcorresponds to the lateral acceleration Rg from one region (Table 2R) ofthe internal ROM.

FIG. 7b shows the content of the table 2R. The roll target value Rtwhich corresponds to the lateral acceleration Rg acts in a direction tocancel the roll which would be developed as a result of the lateralacceleration Rg. The purpose of a region a is to increase the targetroll as the lateral acceleration Rg increases, thereby achieving a powersavings. The purpose of a region b is to prevent any abnormality of asensor which responds to an abnormal value of Rg from providing a rolltarget value even though no Rg is developed in actuality, by reducingthe roll target value. In other respects, the operation in thissubroutine is similar to the subroutine S50 to calculate the heave errorCH mentioned above.

Specifically, HT, Ht at step S39 may be replaced by RT, Rt; the equationto calculate DHT appearing in step S40 may be replaced by a foregoingequation to calculate DRT; EHT1, EHT2 in step S42 may be replaced byERT1, ERT2; EHT2, HT, DHT in step S42 may be replaced by ERT2, RT, DRT;ITH1, ITH2 in step S43 may be replaced by ITR1, ITR2; the equation tocalculate ITh in the subroutine S44 may be replaced by a correspondingequation to calculate a roll error correction ITr; the table 3H may bereplaced by a coefficient table (3R) which is used to calculate the rollcorrection ITr; the table 4H may be replaced by a coefficient table (4R)which is used to calculate the roll correction ITr; ITH2, ITh appearingin step S46 may be replaced by CR, Kr6, ITr. In this manner, a flowchart indicating the subroutine S51 to calculate the roll error CR indetail may be obtained, and is executed by the CPU 17.

The CPU 17 then executes the subroutine S53 to calculate the warp errorCW. The warp error correction CW is calculated in the similar manner asthe heave error CH is calculated, and is then written into warp errorregister CW. A warp target value Wt corresponding to the heave targetvalue Ht is chosen to be equal to zero. In other respects, the operationin this subroutine is similar to the subroutine S50 to calculate theheave error CH mentioned above. Specifically, HT, Ht in step S39 may bereplaced by ET, O; the equation to calculate DHT at step S40 may bereplaced by the foregoing equation to calculate DWT; EHT1, EHT2 at stepS41 may be replaced by EWT1, EWT2; the content of step S42 is modifiedto a content which specifies WT to be 0 when the absolute value of DWTis equal to or below a given value Wm or a permissible range, specifiesWT to be -TWT when Wm is exceeded and writes WT into register TWT2;ITH1, ITH2 at step S43 may be replaced by ITW1, ITW2; the equation tocalculate ITh at the subroutine S44 may be replaced by a correspondingequation to calculate the warp error correction ITw; the table 3H may bereplaced by a coefficient table (3W) which is used to calculate the warpcorrection ITw; the table 4H may be replaced by a coefficient table (4W)which is used to calculate the warp correction ITw; ITH2, ITh at stepS45 may be replaced by ITW2, ITw; and CH, Kh6, ITh in step S46 may bereplaced by CW, Kw6, ITw. In this manner, a flow chart which representsthe subroutine S53 to calculate the warp error CW in detail is obtained,and is executed by the CPU 17.

When the heave error correction CH, the pitch error correction CP, theroll error correction CR, the warp error correction CW have beencalculated, the CPU 17 converts these corrections into suspensionpressure correction EHfL (for the suspension 100fL), EHfr (forsuspension 100fr), EHrL (for suspension 100rL) and EHrr (for suspension100rr) for each of the wheels; thus, the suspension pressure correctionis calculated as follows:

    EHfL=KfL·Kh7·(1/4)·(CH-CP+CR+CW)

    EHfr=Kfr·Kh7·(1/4)·(CH-CP-CR-CW)

    EHrL=KrL·Kh7·(1/4)·(CH+CP+CR-CW)

    EHrr=Krr·Kh7·(1/4)·(CH+CP-CR+CW)

Coefficients KfL, Kfr, KrL, Krr are correction coefficients whichcompensate for differences among suspension feed pressures due todifferential lengths of the pipings leading to the suspensions 100fL,100fr, 100rL, 100rr with respect to the line pressure reference 13rm andthe return pressure reference 13rt. Kh7 represents a coefficient whichincreases or decreases a vehicle elevation deviation correction inaccordance with the steering angular velocity Ss, and is read as afunction of the steering angular velocity Ss from one region (table 5)of the internal ROM. It is expected that there will be a greater changein the attitude when the steering angular velocity Ss is high, so thatthe error in the attitude will also increase. Accordingly, thecoefficient Khs is chosen to be substantially proportional to thesteering angular velocity Ss. However, for a steering angular velocitySs which is below a given level, which is chosen to be 50°/msec in thetable 5, a change in the travelling direction will occur very slowly asis a change in the attitude. For a velocity Ss above 50°/msec and below400°/msec, a change in the attitude will occur at a rate which issubstantially proportional to the steering angular velocity Ss. At asteering angular velocity exceeding 400°/msec, a change in the attitudeof the vehicle will occur very rapidly and excessively, and anyexcessive correction which rapidly compensates for such a rapid changein the attitude will degrade the stability of controlling the vehicleelevation. Accordingly, the correction coefficient Kh7 as a function ofthe steering angular velocity Ss is chosen to be a constant value for Ssat or below 50°/msec, to assume a high value which is substantiallyproportional to Ss for a range from 50°/msec to 400°/msec, and againassume a constant value which it assumes at 400°/msec when the velocityexceeds 400°/msec.

Referring to FIGS. 6c and 6d, the subroutine S32 which effects apredictive calculation of pitching/rolling will be described morespecifically. The preceding subroutine S31 to calculate a deviation ofvehicle elevation has served substantially regulating the suspensionpressures, through a feedback control by determining the current vehicleelevation as well as the current attitude of the car body on the basisof the longitudinal and the lateral acceleration in order to maintain aproper attitude of the car body. By contrast, the subroutine S32 whicheffects a predictive calculation of pitching/rolling principally servescontrolling the longitudinal and the lateral acceleration of the carbody, thus suppressing a change in the longitudinal acceleration Pg andthe lateral acceleration Rg of the car body.

initially, the CPU 17 calculates a correction CGT which is used tosuppress a change in the pitch which is caused by a change in thelongitudinal acceleration Pg (in steps S55 to S58). At this end, thecontent of register GPT2 which stores the correction corresponding to Pgduring the previous pass is written into register GPT1 (in step S55),and a correction Gpt corresponding to Vs and Pg is read from one region(table 6) of the internal ROM and is written into register GPT2 (in stepS57). Data Gpt from the table 6 is specified for groups of Vs as index.Accordingly, the CPU 17 initially specifies a particular group of Vs,and then reads data Gpt corresponding to Pt in a specified group. Foreach group, a deadband a has a width, as indicated by the horizontalwidth where Gpt0=0 in the table 6 shown in FIG. 6c, which is greater fora group having lesser values of Vs. In a region b, the gain is increasedwith an increase in the longitudinal acceleration Pg, thus enhancing thecontrol performance. In a region c, the control performance issuppressed because of the likelihood than an abnormality is occurringwith a sensor or sensors.

The CPU 17 then calculates the correction CGP which is used to suppressa change in the longitudinal acceleration Pg according to the followingequation, and writes it into register CGP (in step S58):

    CGP=Kgp3·[Kgp1·GPT2+Kgp2·(GPT2-GPT1)]

GPT2 represents the content of register GPT2, and represents thecorrection Gpt which is now read from the table 6. GPT1 represents thecontent of register GPT1, and is the correction which was read from thetable 6 during the previous pass. P (proportional) term Kgp1·GPT2 has afactor of proportionally Kgp1.

D (differential) term Kgp2·(GPT2-GPT1) has a coefficient Kgp2, which isread from one region (table 7) of the internal ROM in correspondence tothe vehicle speed Vs. "Table 7" shown in FIG. 6c illustrates generallythat the coefficient Kgp2 assumes a greater value for a greater value ofthe vehicle speed Vs, thus increasing the significance of thedifferential term. This is because the differential term represents acorrection which tends to suppress rapidly a change in the longitudinalacceleration Pg. This is because since the greater the vehicle speed,any change which occurs in the longitudinal acceleration pg as a resultof a depression or release of a brake pedal, the operation of anaccelerator pedal or a turning or turn back of a steering wheel occursmore rapidly, and hence such rapid change must be suppressed rapidly. Onthe other hand, for a vehicle speed Vs which exceeds a certain value,the depression or release of a brake pedal, the operation of anaccelerator pedal or a turning or turn back of a steering wheel, ifallowed to take place rapidly, will cause a very rapid and excessivechange in the longitudinal acceleration Pg, and an excessivedifferential term which provides a rapid suppression of such rapidchange will degrade the stability of suppressing the longitudinalacceleration. Accordingly, the coefficient Kgp2 in the table 7 is chosento undergo a large variation when the vehicle speed is low and assumes aconstant value at and above a given value of the vehicle speed Vs. Inthis manner, the significance of the differential term changes greatlyin response to a change in the vehicle speed, but the significance ofthe differential term ceases to change in response to a change in thevehicle speed when the vehicle speed Vs is high.

The calculated correction CGP which is used to suppress a change in thelongitudinal acceleration Pg represents a pitch correction with respectto the suspension, and Kgp3 represents a weighting factor applied toroll corrections CGR which will be described later.

The CPU 17 then calculates a correction CGR which is used to suppress achange in the roll as caused by a change in the lateral acceleration orthus to suppress a change in the lateral acceleration Pg (steps 59 to62). The content of register GRT2 which stores the correctioncorresponding to Rg which is obtained during the previous pass iswritten into register GRT1 (in step S59), and a correction Grtcorresponding to Vs and Rg is read from one region (table 8) of theinternal ROM and is written in register GRT2 (in step S61). The data Grtin the table 8 is specified for groups of Vs as an index. Accordingly,the CPU 17 specifies a particular group of Vs, and then reads data Grtcorresponding to Rg within the specified group. Each group has adeadband a having a width, as indicated by the lateral dimension of theline representing Grt=0 in the table 8 shown in FIG. 6c, which isgreater for a group of Vs habing lesser values. The purpose of a regionb is to encrease the gain with an increased in the lateral accelerationRg, thus enhancing the control performance. In a region c, the controlperformance is suppressed because of the likelihood than an abnormalityis occurring with a sensor or sensors.

The CPU 17 then calculates a correction CGR which is used to suppress achange in the lateral acceleration Rg according to the followingequation, and writes it into register CGR (in step S62):

    CGR=Kgr3·[Kgr1·GRT2+Kgr2·(GRT2-GRT1)]

GRT2 represents the content of register GRT2, and is a correction Grtwhich is read from the table 8 during the current pass. GRT1 is thecontent of register GRT1, and is a correction which was read from thetable 8 during the previous pass. P (proportional) term Kgr1·GRT2 has afactor of proportionally Kgr1.

D (differential) term Kgr2·(GRT2-GRT1) has a coefficient Kgr2, which isread from one region (table 9) of the internal ROM in correspondence tothe vehicle speed Vs. Generally, the coefficient Kgr2 has a greatervalue for a greater value of vehicle speed Vs, thus increasing thesignificance of the differential term. This is because the differentialterm represents a correction to suppress a change in the lateralacceleration Rg rapidly. It is desired to suppress any rapid change inthe lateral acceleration Rg caused by a turning or turn back of asteering wheel for higher values of the vehicle speed. On the otherhand, for a vehicle speed Vs exceeding a certain value, the lateralacceleration Rg will change rapidly and excessively, if a turning orturn back of a steering wheel takes place rapidly. Any excessivedifferential term which is effective to provide a rapid suppression ofsuch rapid change will degrade the stability of the suppressing of thelateral acceleration. Accordingly, the coefficient Kgr2 in the table 9is chosen to undergo a large variation for low values of the vehiclespeed Vs, and assumes a constant value for vehicle speed Vs whichexceeds a given value. In this manner, the significance of thedifferential term changes greatly in response to a change in the vehiclespeed when the vehicle speed Vs is low, but ceases to change when thevehicle speed Vs is higher.

The calculated correction CGR represents a roll correction for thesuspension and Kgr3 represents a weighting factor applied to pitchcorrection CGP mentioned above and a roll correction GES which will bedescribed later. Since the rate of change in the lateral acceleration Rgis low for low values of the vehicle speed Vs, the specific contributionof the roll correction CGR is reduces in a low speed range while theroll correction is given value in a high speed range. The coefficientdata Kgr3 is stored as a function of the vehicle speed Vs in one region(table 10) of the internal ROM so as to achieve such relationship. TheCPU 17 reads the coefficient Kgr3 corresponding to the speed Vs for usein the calculation of the correction CGR.

The lateral acceleration Rg changes in response to a change in thesteering position or the steering angular velocity Ss, and the rate atwhich such change occurs also depends on the vehicle speed Vs. In otherwords, a change in the lateral acceleration Rg depends on both thesteering angular velocity Ss and the vehicle speed Vs. Accordingly, aroll correction Ges which is required to suppress such change is writteninto one region (table 11) of the internal ROM of the CPU 17. Referringto FIG. 6d, a roll correction Ges corresponding to a particularcombination of Vs and Ss is read from the table 11, and is written intoregister GES (in step S65). If it is substantially equal to zero,indicating that the steering angular velocity of the previous pass isequal to that of the current pass, the roll correction Ges read duringthe previous pass may be used as the roll correction for the currentpass, and hence no updating of register GES (in step S65) takes place.

The CPU 17 then converts the calculated pitch correction CGP, rollcorrection CGR and the roll correction GES into the pressure correctionsfor each of the suspensions, and adds such pressure corrections to thevalues EHfL, EHfr, EHrL, EHrr (the content of registers EHFL, EHFR,EHRR) which have been calculated during the subroutine S31, and utilizetheir sums EhfL, EhrL, Ehrr to update registers EHfL, EHfr, EHrL, EHrr(in step S66).

    EhfL=EHfL+KgfL·(1/4)·(-CGP+Kcgrf·CGR+KgefL.multidot.GES)

    Ehfr=EHfr+Kgfr·(1/4)·(-CGP-Kcgrf·CGR-Kgefr.multidot.GES)

    EhrL=EHrL+KgrL·(1/4)·(CGP+Kcgrr·CGR-KgerL.multidot.GES)

    Ehrr=EHrr+Kgrr·(1/4)·CGP-Kcgrr·CGR+Kgerr.multidot.GES)

The first term on the right side represent values calculated during thesubroutine S31 which were written into registers EHfL, EHfr, EHrL, EHrrwhile the second term on the right side represents the pitch correctionCGP, the roll correction CGR and the roll condition GES mentioned abovewhich have been converted into pressure corrections for the respectivesuspensions. The coefficients KgfL, Kgfr, KgrL and Kgrr appearing in thesecond term of the right hand are related as follows:

    KgfL=KfL·Kgs

    Kgfr=Kfr·Kgs

    KgrL=KrL·Kgs

    Kgrr=Krr·Kgs

KfL, Kfr, KrL, Krr are coefficients which are used to correct pressureerrors caused by variations among the piping length leading to therespective suspension with respect to the pressure reference while Kgsrepresents a coefficient which has a predetermined relationship with thesteering angular velocity Ss as indicated in the table 12 and whichdefines a weighting factor applied to the pressure correction which iscalculated in the subroutine S32 and used to suppress a change in theacceleration such as the second term in the right of above fourequations,

    (1/4)·(-CGP+KcgrL·CGR+KgefL·GES),

relative to the pressure correction calculated in the subroutine S31.Since it is expected that a change in the acceleration will occurrapidly for a greater value of the steering angular velocity Ss, it ispreferred to choose a greater weighting factor for a pressure correctionwhich is used to suppress a change in the acceleration. Accordingly, thecoefficient Kgs is generally chosen to be greater in proportion to thesteering angular velocity Ss. However, for a steering angular velocitySs, at or below a given level, which is chosen to be 50°/msec in thetable 12, a change in the acceleration will be minimal. For a range from50°/msec to 400°/msec, the acceleration will change at a rate which issubstantially proportional to the steering angular velocity Ss. For asteering angular velocity exceeding 400°/msec, a change in the turningradius will occur rapidly and excessively, causing an excessively largechange in the acceleration in particular in the lateral acceleration. Anexcessive correction which provides a rapid compensation of such a rapidchange in the acceleration will degrade the stability of controlling theacceleration. Accordingly, the weighting factor Kgs is chosen as afunction of the velocity Ss. Specifically, it assumes a constant valuefor Ss at or below 50°/msec, and assumes a higher value which issubstantially proportional to Ss for a range from 50°/msec to 400°/msec,and again assumes a constant value which is equivalent to the valuereached at 400°/msec when 400°/msec is exceeded.

The CPU 17 then adds the initial pressure data which are stored in theinitial pressure registers PFLo, PFRo, PRLo, PRRo and which have beendetermined at steps S16 to S18 to the sum of the correction pressureused to regulate a deviation in the vehicle elevation and the correctionpressure used to control suppressing the acceleration which iscalculated in a subroutine S66 (the content of registers EHfL, EHfr,EHrL, EHrr), thus determining the pressures to be applied to theindividual suspensions and updating registers EHfL, EHfr, EHrL, EHrrwith these values (in step S67).

Referring to FIG. 6e, the subroutine S33 which effects "pressurecorrection" will be described. The CPU 17 reads a correction PH, whichis designed to compensate an error or a deviation of the output pressureof the pressure control valves due to a change or a fluctuation of theline pressure, corresponding to the pressure Dph (the content ofregister DPH) detected by the pressure sensor 13rm from a region (table13H shown in FIG. 6e) of the internal ROM and reads a correction PLf forfront wheel side and a correction PLr for rear wheel side, which aredesigned to compensate and error or a deviation of the output pressureof the pressure control valves due to a change or a fluctuation of thereturn pressure, corresponding to the pressure DpL (the content ofregister DPL) detected by the pressure sensor 13rt from a region (table13L shown in FIG. 6e) of the internal ROM. Then the CPU 17 calculatespressure correction values PDf=Ph-PLf (for front wheel side) andPDr=PH-PLr (for rear wheel side) for compensating the errors of theoutput pressure of the pressure control valves due to the change orfluctuation of the line pressure and the return pressure (in steps S68and S69). Since the distance from the reservoir 2 to the suspensions offront wheel side is shorter than that from the reservoir 2 to thesuspensions of rear wheel side, since the return pressure sensor 13rtdetects the return pressure at the rear wheel side and since the returnpressure difference between the front wheel side and the rear wheel sideis relatively high, the pressure corrections of each suspensionresponding to the return pressure is separated as described for reducingerror of the pressure compensation. As indicated by "table 13L" in FIG.6e, it includes two groups of correction values, one of which isassigned to the front wheel side and the other to the rear wheel side.The CPU 17 reads a correction value which corresponds to the returnpressure detected by the sensor 13rt from the group assigned to thefront wheel side for the suspensions of the front wheel side and reads acorrection value from the other group assigned to the rear wheel sidefor the suspensions of the rear wheel side.

After the calculation of the corrections PDf and Pdr, the CPU 17 addseach one of them to the contents in the registers EHfL, EHfr and EHrL,EHrr and writes the resulting sums into the registers HfL, EHfr, EHrL,EHrr (subroutine S71).

Referring to FIG. 6f, the subroutine 34 for "pressure-currentconversion" will be described in detail. The CPU 17 reads current levelsIhfL, Ihfr, IhrL, Ihrr which are to be applied to the pressure controlvalves 80fL, 80fr, 80rL and 80rr, respectively, in order to produce thepressures indicated by data EHfL, EHfr, EHrL and EHrr of registers EHfL,EHfr, EHrL, EHrr from pressure/current conversion table 1b, and writesthem into current output registers IHfL, IHfr, IHrL and IHrr (in stepS34).

Referring to FIG. 6g, the subroutines S35 for "warp correction" will nowbe described. In the subroutine 35, an appropriate target warp DWT iscalculated on the basis of the lateral acceleration Rg and the steeringangular velocity Ss (in step S73), and a warp which would result whenthe content of registers IHfL, IHfr, IHrL, IHrr is delivered is alsocalculated. Its error warp with respect to the target warp DWT iscalculated (in step S77), and current corrections dIfL, dIfr, dIrL, dIrrwhich are required to reduce the error warp to zero are calculated (instep S77), and these current corrections are added to the content ofregisters IHfL, IHfr, IHrL, IHrr, with these registers updated by theresulting sum (in step S78).

One region (table 14) of the internal rom of the CPU 17 has warp targetvalues Idr stored therein in a manner corresponding to the magnitude ofthe lateral acceleration Rg. Also table 15 has warp target values Idsstored therein which correspond to the steering angular velocity Ss.Table 16 has warp corrections Idrs stored therein which correspond tothe inclination of the car body in the fore-and-aft direction as well asthe lateral acceleration Rg (representing the inclination in the lateraldirection) as defined by the values in registers IHfL, IHfr, IHrL, IHrrwhich are to be delivered. It is to be understood that the inclinationin the fore-and-aft direction is indicated as follows:

    K=|(IhfL+Ihfr)/(IhrL+Ihrr)|

The Table 16 includes groups of data corresponding to differentmagnitudes of K, and data in each group is given in correspondence tothe lateral acceleration Rg.

The CPU 17 reads warp target value Idr corresponding to the lateralacceleration Rg from the table 14, reads warp target value Idrcorresponding to the steering angular velocity Ss, and reads the warpcorrection Idrs corresponding to the inclination in the fore-and-aftdirection as well as in the lateral direction, as defined by the valuesin registers IHfL, IHfr, IHrL, IHrr from the table 16, and calculateswarp target value DWT as follows (in step S73):

    DWT=Kdw1·Idr+Kdw2·Ids+Kdw3·Idrs

The CPU 17 then calculates a warp, which is defined as follows:

    (IhfL-Ihfr)-(IhrL-Ihrr)

as defined by the content of registers IHfL, IHfr, IHrL, IHrr, andexamines if it remains within a permissible range or deadband (in stepS74). If it is out of the permissible range, the calculated warp(IhfL-Ihfr)-(IhrL-Ihrr) is subtracted from the target warp DWT, and theresult is written into a warp error correction register DWT (in stepS75). If it remains within the permissible range, the content (DWT) ofthe register DWT remains unchanged. The warp error correction DWT (thecontent of register DWT) is multiplied by the weighting factor Kdw4 toprovide a product, which is used to update register DWT (in step S76).The warp error correction DWT is converted into a correction for each ofthe suspension pressures, or more exactly, a correction to be applied tothe current setting of the respective pressure control valve whichcorresponds to the pressure correction (in step S77), and the resultingcorrection is added to the content of current output registers IHfL,IHfr, IHrL, IHrr (in step S78).

Data from these current output registers IHfL, IHfr, IHrL, IHrr aretransferred to the CPU 18 so as to be supplied to the pressure controlvalves 80fL, 80fr, 80rL, 80rr during the output subroutine S36 (FIG.5c), and the CPU 198 applies them to the duty controller 32.

Referring to FIGS. 6h and 6i, the subroutine CTS for "test" will bedescribed in detail. Note that the subroutine "test" (CTS) is executedwhen the test indication switches TSW1 and TSW2 are closed and thevehicle speed Vs is lower than the predetermined value A and that it isexecuted repeatedly substantially with the ST period through a loopCTS-36-37-19-20-LPC-22A to 22D-CTS (FIGS. 5c and 5b).

In the "test" subroutine CTS, at first, the CPU 17 reads a content Sp ina steering angle register SP and determines a region in which thesteering position or angle Sp belongs (in steps S80, S89 and S90).Steering angle regions are as follows:

    Sp≧B,

    B>Sp≦C,

    C>Sp≦D,

    D>Sp,

wherein B>C>D.

The CPU 17 executes "determination of roll/pitch of ±E" (in subroutineS81) when Sp≧B, "determination of roll/pitch of ±F" (in subroutine S91)when B>Sp≦C, "determination of roll/pitch of ±G" (in subroutine S92)when C>Sp≦D or "determination of roll/pitch of ±H" (in subroutine S93)when D>Sp, wherein E>F>G>H.

In the "determination of roll/pitch of ±E" (in subroutine S81) whenSp≧B, the CPU 17 examines if the height indication switch HSW indicates"high" (which indicates rolling in this case) or not (in step S82), andif the switch HSW indicates "high", the CPU 17 examines a content in adirection register (in step S83). If the contents in the directionregister is "L" (which means that the steering wheel is rotted towardright), this means that the steering wheel is rotated toward right by Bor more. Thus the CPU 17, for lowering the right side of the vehicleremarkably and for rising up the left side of the vehicle remarkably,writes into target elevation registers DFLt and DRLt assigned to frontand rear, left wheels a value greater than a reference value Htln (whichis a quarter of Htln shown in the table 2H in FIG. 6a) by E, and CPU 17writes into target elevation registers DFRt and DRRt assigned to frontand rear, right wheels a value smaller than the reference value Ht In byE (step 85). If the content in the direction register is "H" (whichmeans that the steering wheel is rotated toward left), this means thatthe steering wheel is rotated toward left by B or more. Thus CPU 17, forrising up the right side of the vehicle remarkably and for lowering theleft side of the vehicle remarkably, writes into target elevationregisters DRFt and DRRt assigned to front and rear, right wheels a valuegreater than the reference value HtIn by E, and CUP 17 writes intotarget elevation registers DFLt and DRLt assigned to front and rear,left wheels a value smaller than the reference value HtIn by E (step86).

If the height indication switch HSW indicates "normal"(which indicatespitching in this case) and if the switch HSW indicates "high", CPU 17examines the content in the direction register (step 84). If the contentin the direction register is "L" (which means that the steering wheel isrotated toward right), CPU 17, for lowering the front side of thevehicle remarkably and for rising up the rear side of the vehicleremarkably, writes into target elevation registers DFLt and DFRtassigned to left and right, front wheels a value smaller than thereference value HtIn by E, and the CPU 17 writes into target elevationregisters DFLt and DFRt assigned to left and right, rear wheels a valuegreater than the reference value HtLn by E (in step S87). If the contentin the direction register is "H" (which means that the steering wheel isrotated toward left), this means that the steering wheel is rotatedtoward left by B or more. Thus the CPU 17, for rising up the front sideof the vehicle remarkably and for lowering the rear side of the vehicleremarkably, writes into target elevation registers DFLt and DFRtassigned to left and right, front wheels a value greater than thereference value HtIn by E, and CPU 17 writes into target elevationregisters DRLt and DRRt assigned to left and right, rear wheels a valuesmaller than the reference value HtIn by E (in step S88).

The CPU 17 executes "determination of roll/pitch of ±F" (in subroutineS91) when B>Sp≦C, "determination of roll/pitch of ±G" (in subroutineS92) when C>Sp≦D or "determination of roll/pitch of ±H" (in subroutineS93) when D>Sp in a similar manner as described in "determination ofroll/pitch of ±E" (in subroutine S81) shown in FIG. 6h. However, Eshould be replaced by F, G or H respectively.

After the execution of subroutine S81, S91, S92 or S93, the CPU 17executes "pressure correction for suspension 100fL" (in subroutine S94),"pressure correction for suspension 100fr" (in subroutine S98),"pressure correction for suspension 100rL" (in subroutine S99) and"pressure correction for suspension 100rr" (in subroutine S100)successively in this order.

In the "pressure correction for suspension 100fL" (in subroutine S94),the CPU 17 examines if the detected height DFL at suspension 100fL isequal with or higher than the target value DFLt in the target elevationregister DFLt (step 95), the CPU 17 decreases the pressure correctionEHfL by one step (a predetermined small value) (in step S97) when theheight DFL is equal with or greater than the target value DFLt. The CPU17 increases the pressure correction EHfL by one step (in step S96) whenthe height DFL is smaller than the target value DFLt.

The CPU 17 executes "pressure correction for suspension 100fr" (insubroutine S98), "pressure for suspension 100rL" (in subroutine S98) and"pressure correction for suspension 100rr" (in subroutine S100) in asimilar manner as described in "pressure correction for suspension100fL" (in subroutine S94) shown in FIG. 6i. However, DFL should bereplaced by DFR, DRL and DRR respectively, DFLt should be replaced byDFRt, DRLt and DRRt respectively, and EHfL should be replaced by EHfr,EHrL and EHrr respectively.

After the execution of pressure correction of suspensions (insubroutines S94, S98, S99 and S100), CPU 17 adds the initial pressuredata PFLo, PFRo, PRLo and PRRo to the pressure correction data EHfL,EHfr, EHrL and EHrr respectively, then reads current levels IhfL, Ihfr,IhrL and Ihrr which are to be applied to the pressure control valves80fL, 80fr, 80rL and 80rr respectively in order to produce the pressuresindicated by EHfL, EHfr, EHrL and EHrr from pressure/current conversiontable 1, and writes them into current registers IHfL, IHfr, IHrL andIHrr (the subroutine S101). Data from these current output registersIHfL, IHfr, IHrL, IHrr are transferred to the CPU 18 so as to besupplied to the pressure control valves 80fL, 80fr, 80rL, 80rr duringthe output subroutine S36 (FIG. 5c), and the CPU 18 applies them to theduty controller 32.

Since "test" (subroutine CTS) is executed repeatedly substantially withthe ST period through a loop CTS-36-37-19-20-LPC-22A to 22D-CTS (FIGS.5c and 5b), the height DFL, DFR, DRL and DRR at suspensions 100fL,100fr, 100rL and 100rr become substantially equal to the target valuesDFLt, DFRt, DRLt and DRRt which are determined in the "determination ofpitch/roll" (in subroutines S81, S91, S92 or S93).

Thus, if the height indication switch HSW indicates "high" and thesteering wheel rotates toward right by B or more, the vehicle rollstoward right, namely the right side of the vehicle falls under areference level by E and the left side of the vehicle rises over thereference level by E. If the steering wheel rotates toward left, thevehicle rolls in inverse direction. If the rotation Sp of the steeringwheel is B>Sp≧C, the vehicle rolls such that one of the right and leftsides rises by F and the other side falls by F. If the rotation Sp ofthe steering wheel is C>Sp≧D, the vehicle rolls such that one of theright and left sides rises by G and the other side falls by G. However,the rolling of the vehicle do not appear when D>Sp (providing that H=0).

If the height indication switch HSW indicates "normal" and the steeringwheel rotates toward right by B or more, the vehicle pitches toward thefront side, namely the front side of the vehicle falls under a referencelevel by E and the rear side of the vehicle rises over the referencelevel by E. If the steering wheel rotates toward left, the vehiclepitches in inverse direction. If the rotation Sp of the steering wheelis B>Sp≧C, the vehicle pitches such that one of the front and rear sidesrises by F and the other side falls by F. If the rotation Sp of thesteering wheel is C>Sp≧D, the vehicle pitches such that one of the frontand rear sides rises by G and the other side falls by G. However, thepitching of the vehicle do not appear when D>Sp (providing that H=0).

According to the embodiment described above, an initial suspensionpressure control (in subroutines S25, S31 to S36) is executed by CPU 17when the ignition key switch 20 and the pressure control indicationswitch are both closed by the driver, providing that at least one of thetest indication switches TSW1 and TSW2 is open. In this initialsuspension pressure control, the height of the vehicle is determined toa standard height by the CPU 17 when the height indication switch HSWindicates "normal", whereas the height of the vehicle is determined to aheight which is higher than the standard height when the heightindication switch HSW indicates "high". Accordingly, the height of thevehicle goes up or down by switching the height indication switch HSWfrom "normal" to "high" or from "high" to "normal". Thus the driver cantest if the pressure control system correctly change the height (heave)responding to the switching of the height indication switch HSW. Duringa running of the vehicle, the pressure of each one of the suspensions iscontrolled responding to the rotation of the steering wheel so as tosuppress a rolling of the vehicle which might be arisen by the rotationof the steering wheel and responding to a longitudinal acceleration anda lateral accelerations so as to suppress a pitching and a rolling ofthe vehicle which might be arisen by the accelerations.

The driver, when he intends to test the attitude control of the pressurecontrol system, stops the vehicle and closes the test indicationswitches TSW1 and TSW2. Then he indicates "high" or "normal", forexample "high" which indicates a test of rolling, by the heightindication switch HSW and rotates the steering wheel by B toward right(a test of rightward rolling). Then the vehicle laterally inclines suchthat the height of the left side of the vehicle rises up and the rightside falls down corresponding to the rotation of the steering wheel. Theinclination increases responding to the increase of the rotation of thesteering wheel. The inclination decreases when the steering wheelrotates inverse (leftward) direction toward the neutral position. Whenthe steering wheel rotates beyond the neutral position toward left (atest of leftward rolling), the vehicle laterally inclines such that theheight of the left side of the vehicle falls down and the right siderises up corresponding to the rotation of the steering wheel. Theinclination increases responding to the increase of the rotation of thesteering wheel. The inclination decreases when the steering wheelrotates inverse (rightward) direction toward the neutral position. Thenthe inclination becomes substantially zero at the neutral position.

If the drive indicates "normal" which indicates a test of pitching bythe height indication switch HSW and rotates the steering wheel by Btoward right (a test of frontward pitching). Then the vehicle pitchessuch that the height of the rear side of the vehicle rises up and thefront side falls down corresponding to the rotation of the steeringwheel. The pitching inclination increases responding to the increase ofthe rotation of the steering wheel. The inclination decreases when thesteering wheel rotates inverse (leftward) direction toward the neutralposition. When the steering wheel rotates beyond the neutral positiontoward left (a test of rearward pitching), the vehicle pitches such thatthe height of the rear side of the vehicle falls down and the front siderises up corresponding to the rotation of the steering wheel. Thepitching inclination increases responding to the increase of therotation of the steering wheel. The pitching inclination decreases whenthe steering wheel rotates inverse (rightward) direction toward theneutral position. Then the pitching inclination becomes substantiallyzero at the neutral position.

If a pressure control for the attitude control responding to therotation of the steering wheel should fail, the rolling and the pitchingas described do not arise. Also, if a pressure control for the attitudecontrol responding to the longitudinal acceleration and the lateralacceleration should fail, the rolling and the pitching as described dono arise in a predetermined inclination mode.

If a pressure control of a suspension, for example the suspension 100frfor front, right wheel, should fail, the height at front, right side ofthe vehicle do no change substantially during the test as described,which causes an unexpected pitching or rolling when the heightindication switch HSW indicates "high" (test of rolling) or "normal"(test of pitching) respectively.

If a pressure control of two suspensions should fail, the rolling asdescribed do not arise between the two suspensions when the twosuspensions are 100fr, ; 100fL or 100rr, 100rL, the pitching asdescribed do not arise between the two suspensions when the twosuspensions are 100fr, 100rr or 100fL, 100rL, or an unexpected pitchingor rolling arises when the two suspensions are 100fL, 100rr or 100fr,100rL. If a pressure control of three or four suspensions should fail,the rolling and the pitching as described do no arise.

If the driver turns at least one of the test indication switches TSW1and TSW2 to open state, or the driver starts the vehicle to runremaining the switches TSW1 and TSW2 closed and then the vehicle speedVs become to or greater than A, the CPU 17 stops the "test" subroutineCTS and starts the execution of the subroutines "calculate parameter"(S25), "calculate deviation in elevation" (S31), "predictive calculationof pitching/rolling"(S32) and etc. for regulating or adjusting theheight and the attitude of the vehicle responding to getting on or off aperson to the vehicle, running condition of the vehicle on a road ordriving operation of the driver to the vehicle.

While a preferred embodiment of the invention have been illustrated anddescribed, it is to be understood that here is no intention to limit theinvention to the precise constructions disclosed herein and the right isreserved to all changes and modifications coming within the scope of theinvention as defined in the appended claims.

What we claimed is:
 1. A pressure control system for suspensioncomprising:pressure determining devices, respective one of whichdetermines, responding to a pressure instruction, a pressure ofrespective one of suspensions on a vehicle; sensors for detectingrunning conditions of the vehicle; an electronic controller forcalculating pressures responding to the running conditions detected andinstructing pressures to respective one of the pressure determiningdevices; test indication means for indicating a test control of thesuspensions to the electronic controller; a test pressure indicatorwhich indicates a test pressure for the suspensions to the electroniccontroller; and said electronic controller, responding to the indicationof the test control from the test indication means, generates a pressureinstruction for causing a pressure difference, which corresponds to thetest pressure indicated by the test pressure indicator, between thesuspensions at the front side and the rear side.
 2. A pressure controlsystem as defined in claim 1, wherein the sensors for detecting runningconditions of the vehicle include vehicle elevation sensors respectiveone of which detects a height of the vehicle at respective one of thesuspensions, a longitudinal acceleration sensor which detects anacceleration in longitudinal direction of the vehicle, a lateralacceleration sensor which detects a lateral acceleration of the vehicleand a rotation sensor which detects a rotational position of a steeringwheel on the vehicle and the test pressure indicator is the rotationsensor.
 3. A pressure control system for suspension comprising:pressuredetermining devices, respective one of which determines, responding to apressure instruction, a pressure of respective one of suspensions on avehicle; sensors for detecting running conditions of the vehicle; anelectronic controller for calculating pressures responding to therunning conditions detected and instructing pressures to respective oneof the pressure determining devices; test indication means forindicating a test control of the suspension to the electroniccontroller; a test pressure indicator which indicates a test pressurefor the suspensions to the electronic controller; and said electroniccontroller, responding to the indication of the test control from thetest indication means, generates a pressure instruction for causing apressure difference, which corresponds to the test pressure indicated bythe test pressure indicator, between the suspensions at the right sideand the left side.
 4. A pressure control system as defined in claim 3,wherein the sensors for detecting running conditions of the vehicleinclude vehicle elevation sensors respective one of which detects aheight of the vehicle at respective one of the suspensions, alongitudinal acceleration sensor which detects an acceleration inlongitudinal direction of the vehicle, a lateral acceleration sensorwhich detects a lateral acceleration of the vehicle and a rotationsensor which detects a rotational position of a steering wheel on thevehicle and the test pressure indicator is the rotation sensor.
 5. Apressure control system for suspension comprises:pressure determiningdevices, respective one of which determines, responding to a pressureinstruction, a pressure of respective one of suspensions on a vehicle;sensors for detecting running conditions of the vehicle; an electroniccontroller for calculating pressures responding to the runningconditions detected and instructing pressures to respective one of thepressure determining devices; test indication switches for indicatingtest controls of the suspensions to the electronic controller; a testpressure indicator which indicates test pressures for the suspensions tothe electronic controller; said electronic controller, responding to anindication of a first test control from the test indication switches,generates a first pressure instruction for causing a pressuredifference, which corresponds to a test pressure indicated by the testpressure indicator, between the suspensions at the front side and therear side; and said electronic controller, responding to an indicationof a second control from the test indication switches, generates asecond pressure instruction for causing a pressure difference, whichcorresponds to a test pressure indicated by the test pressure indicator,between the suspensions at the right side and the left side.
 6. Apressure control system as defined in claim 5, wherein the sensors fordetecting running conditions of the vehicle include vehicle elevationsensors respective one of which detects a height of the vehicle atrespective one of the suspensions, a longitudinal acceleration sensorwhich detects an acceleration in longitudinal direction of the vehicle,a lateral acceleration sensor which detects a lateral acceleration ofthe vehicle and a rotation sensor which detects a rotational position ofa steering wheel on the vehicle and the test pressure indicator is therotation sensor.